The invention relates to a process for recycling waste sulfuric acids by the decomposition of organic contaminants using light energy.
Sulfuric acid is used as a reactant or auxiliary material in numerous processes in the chemical industry and in mineral oil refining. In the course of the respective process, a large amount of the sulfuric acid employed is frequently converted into waste sulfuric acid.
The most important basic processes of organic-chemical technology using sulfuric acid, oleum and/or sulfur trioxide include, for example, sulfonations, sulfochlorinations, sulfations, exchange reactions of sulfonate groups, nitrations, hydrations of olefins to form alcohols and ethers, processes for producing polyamide precursors, processes for producing methacrylic acid esters and of other organic acids or derivatives thereof, digestion processes and processes for treating waste liquors in the pulp and paper industry, processes for saccharification of starch- and cellulose-containing materials as well as for the production of furfurol and gelatin, processes for the production of nitrocellulose and of explosives, processes for refining lubricants, regeneration of used oils, waxes, tar oils and of crude benzene, as well as general alkylation processes in the mineral oil industry.
Waste sulfuric acid is frequently contaminated with reaction by-products or diluted materials and possibly with microparticles. Alternatively, the sulfuric acid is diluted with water to such a degree that it can no longer be utilized for technical purposes and will have to be discarded. For reasons of environmental protection, disposal mechanisms such as the dumping of waste sulfuric acid into the open sea or into landfills are no longer tolerable and also are no longer permitted by an increasing number of countries. Thus, waste sulfuric acid must be recycled to give a reusable fresh acid.
To date, the prior art follows two different approaches:
1) Concentrating, and if desired or required, concentrating to a high degree of waste sulfuric acid which is frequently obtained having concentrations within the range of 20-70%; and PA1 2) high-temperature recycling of a highly contaminated waste sulfuric acid by decomposing it into water, sulfur dioxide and oxygen (cf. Winnacker/Kichler, Chemische Technologie, 4th edition, vol. 2, page 1 et seq.).
The concentrating process hitherto has been used primarily for the purification of a diluted waste sulfuric acid containing a low level of contaminants, where the contaminants are readily removable or where the purity requirements for the regenerate are not particularly high (such as, for example, for sulfuric acid employed for the development of ilmenite in the production of titanium dioxide). If, however, the regenerate is to meet higher requirements or a highly contaminated waste sulfuric acid is intended to be recycled by concentration, then additional cost-intensive purification procedures are required. These include, especially in the case of waste sulfuric acids bearing organic contaminants, the use of strong oxidants such as hydrogen peroxide or nitric acid. Furthermore, concentrating procedures are employed for the preconcentration of highly diluted acids, before the acids are introduced into the cracking plant. When the amount of sulfuric acid is being concentrated to more than 70% by weight, then the sulfuric acid becomes dehydrated, and with further increases in temperature and sulfuric acid concentration, a sulfur trioxide partial pressure forms above the liquid phase. More specifically, the sulfuric acid decomposes above its boiling point in the gaseous phase to form sulfur trioxide and water. This reaction is reversible, so that sulfuric acid may be recovered after cooling.
The high temperature regeneration process via the decomposition of the sulfuric acid molecule is primarily employed in regenerating sulfuric acid which is highly contaminated with organic waste and for those cases either where the regenerate is to meet high demands with respect to its purity or where toxic matter will have to be safely destroyed. Sulfur dioxide, a decomposition product obtained from sulfur trioxide, is usually processed further in a subsequent sulfuric acid plant to form a concentrated fresh acid which is free from impurities.
A particular drawback inherent to the commercially employed processes for recycling waste sulfuric acid is the considerable operating cost to cover the energy requirements and, in the case of the high-temperature cracking, additional capital expenditure plus operating cost for a sulfuric acid plant. Moreover, the energy demand of the high temperature cracking procedure is higher than that of the concentrating procedure. The regeneration of waste sulfuric acid is more expensive than the production of fresh acid from the raw materials sulfur or pyrite. As a matter of fact, a high-temperature cracking plant for regenerating waste sulfuric acid preferably will be established as an interconnected operation with a sulfuric acid plant, wherein fresh acid is produced from the raw materials, so that the feed of raw materials can be reduced. However, it is disadvantageous that part of the capacity of the sulfuric acid plant is committed to the regeneration process. Another drawback of the processes as technically realized today is that the energy demand will regularly have to be satisfied by the combustion of fossil fuel. Nevertheless, plants using direct solar radiation, such as dish and tower systems equipped with reflectors acting to effect an optical concentration and with radiation recipients (receivers), have been developed for solar-thermal power generation in order to reduce the use of fossil carriers of primary energy and to avoid the emissions associated with combustion. Such plants are capable of providing a high-temperature processing heat in excess of 1000.degree. C. O. Weinmann, K. H. Funken, K. F. Knoche, and R. Sizmann; DGS (Ed.) Tagungsbericht 7.Internationales Sonnenforum, Frankfurt, Oct. 9-12, 1990 Vol. 2, pages 1076-1981, proposing to utilize this source also for the decomposition of waste acid. The waste acid is injected into the volumetric receiver and is heated, dehydrated, evaporated and decomposed therein. The waste acid is directly exposed to highly concentrated solar radiation. An open volumetric receiver is chosen as the constructional design. However, a sufficiently high air stream will have to be drawn in so that sulfuric acid and/or reaction products are prevented from escaping into the surroundings. It is assumed that the oxidation of the organic contaminants could be accelerated by the direct irradiation with highly focussed sunlight in a volumetric receiver. A review on volumetric receivers is presented by C. J. Winter, R. L. Sizmann, L. L. Vant-Hull in Solar Power Plants, Springer-Velag, Berlin, Heidelberg, New York, 1991. In accordance therewith, a volumetric receiver consists of a volume space with a multiplicity of porous interconnected shaped articles, wire packs, foam or sheet assemblies made of metal, ceramic or other suitable materials. In volumetric receivers, the concentrated radiation heats the material present in the volume space. At the same time the heat transfer medium flows through the volume and is convectively heated. In order to achieve a high degree of absorption of the radiation, the volume is tightly filled with porous fillings. This results in providing narrow flow channels, a low radiation penetration depth and a relatively short residence time of the fluid medium in the irradiated zone (about 0.01 seconds in foam or sheet receivers). The conversion of the organic contaminants increases with an increase in the residence time. Experimental investigations have shown, however, that longer residence times are required for achieving a high conversion. Thus, volumetric receivers can no longer be considered as being suitable, since the residence time in the irradiated zone is too short.